1. Field of the Invention
The invention relates generally to the field of marine seismic surveying. More specifically, the invention relates to methods for processing signals acquired using streamers having both pressure responsive sensors and motion responsive sensors.
2. Background Art
In seismic exploration, geophysical data are obtained by applying acoustic energy to the earth from an acoustic source and detecting seismic energy reflected from interfaces between different layers in subsurface earth formations. The seismic wavefield is reflected when there is a difference in acoustic impedance between the layer above the interface and the layer below the interface. When using towed streamers in marine seismic exploration, one or more seismic streamers is towed behind an exploration vessel at a water depth typically between about six to about nine meters, but can be towed shallower or deeper. Seismic sensors (also known as seismic receivers) are included in the streamer cable for detecting seismic signals. Typically employed are pressure sensitive sensors, such as hydrophones, and particle motion sensitive sensors, such as geophones. The seismic sensors convert the seismic wavefields into electrical or optical signals that are typically recorded for signal processing, and evaluated to estimate characteristics of the subsurface of the earth.
The resulting seismic data obtained in performing a seismic survey is processed to yield information relating to the geologic structure and properties of the subterranean formations in the area being surveyed. The processed seismic data is processed for display and analysis of potential hydrocarbon content of these subterranean formations. The goal of seismic data processing is to extract from the seismic data as much information as possible regarding the subterranean formations in order to adequately image the geologic subsurface. In order to identify locations in the Earth's subsurface where there is a probability for finding petroleum accumulations, large sums of money are expended in gathering, processing, and interpreting seismic data. The process of constructing the reflector surfaces defining the subterranean earth layers of interest from the recorded seismic data provides an image of the earth in depth or time. The image of the structure of the Earth's subsurface is produced in order to enable an interpreter to select locations with the greatest probability of having petroleum accumulations.
In a typical geophysical exploration configuration, a plurality of streamer cables are towed behind a vessel. One or more seismic sources are also normally towed behind the vessel. The seismic source, which typically is an airgun array, but may also be a water gun array or other type of source known to those of ordinary skill in the art, transmits seismic energy or waves into the earth and the waves are reflected back by reflectors in the earth and recorded by sensors in the streamers. Paravanes are typically employed to maintain the cables in the desired lateral position while being towed. Alternatively, the seismic cables are maintained at a substantially stationary position in a body of water, either floating at a selected depth or lying on the bottom of the body of water, in which case the source may be towed behind a vessel to generate acoustic energy at varying locations, or the source may also be maintained in a stationary position.
After the reflected wave reaches the streamer cable, the wave continues to propagate to the water/air interface at the water surface, from which the wave is reflected downwardly, and is again detected by the hydrophones in the streamer cable. The water surface is a good reflector and the reflection coefficient at the water surface is nearly unity in magnitude and is negative in sign for pressure signals. The waves reflected at the surface will thus be phase-shifted 180 degrees relative to the upwardly propagating waves. The downwardly propagating wave recorded by the receivers is commonly referred to as the surface reflection or the “ghost” signal. Because of the surface reflection, the water surface acts like a filter, which creates spectral notches in the recorded signal, making it difficult to record data outside a selected bandwidth. Because of the influence of the surface reflection, some frequencies in the recorded signal are amplified and some frequencies are attenuated.
A particle motion sensor, such as a geophone, has directional sensitivity, whereas a pressure sensor, such as a hydrophone, does not. Accordingly, the upgoing wavefield signals detected by a geophone and hydrophone located close together will be in phase, while the downgoing wavefield signals will be recorded 180 degrees out of phase. Various techniques have been proposed for using this phase difference to reduce the spectral notches caused by the surface reflection. An alternative to having the geophone and hydrophone co-located, is to have sufficient spatial density of sensors so that the respective wavefields recorded by the hydrophone and geophone can be interpolated or extrapolated to produce the two wavefields at the same location.
In order to properly account for the directional sensitivity of the particle motion sensor, the angle of incidence along the seismic streamers and transverse thereto is required. Therefore, a need exists for a method for combining pressure and particle motion signals that accounts for the angle of incidence for 3-D acquisition geometries.